I meant to post last night about the new name for Microsoft Standalone System Sweeper but got side-tracked. You are correct, Windows Defender Offline is the new name for the Beta.

The purpose of the offline version is to help help start an infected PC and perform an offline scan to help identify and remove rootkits and other advanced malware. It can also be used in situations where an antivirus software fails to install or the program that is installed is unable to detect or remove malware from the computer. The Microsoft Standalone System Sweeper Beta uses the same definitions that are used for Microsoft Security Essentials and Microsoft Forefront.

I suspect the renaming is a result of Microsoft Security Essentials being named Windows Defender for Windows 8.